1. Field of the Invention
The present invention relates to a method and apparatus for providing air with a less than ambient concentration of oxygen (reduced-oxygen air) to a human or other subject. More particularly, the invention relates to a method and apparatus for inducing hypoxia in a subject by delivering enriched nitrogen (and, thereby, reduced-oxygen) air to the subject in an isobaric setting to simulate various altitudes above sea level over relatively short periods.
2. Description of Prior Art
Altitude sickness strikes thousands of individuals every year resulting in problems from sleep disorders to pulmonary edemas to death. These individuals are pilots, skiers, mountain climbers, or merely business travelers to high altitude regions. The key to dealing with the altitude sickness is taking advantage of the body's ability to gradually acclimatize through a transition through progressively higher altitudes. Unfortunately, most individuals do not have the time to acclimatize.
The physiology of altitude sickness and the adjustment to altitude is covered in numerous textbooks. An excellent one is “Medicine For Mountaineering” by James Wilkerson, M.D. Copyright 1992, published by The Mountaineers of Seattle, Wash. from which much of the immediately following discussion is derived.
The body adjusts to altitude by increasing respiratory volume, increasing the pulmonary artery pressure, increasing the cardiac output, increasing the number of red blood cells, increasing the oxygen carrying capability of the red blood cells, and even changing body tissues to promote normal function at lower oxygen levels.
For example, at an altitude level of 3,000 feet the body already begins increasing the depth and rate of respiration. As a result of this, more oxygen is delivered to the lungs. In addition, the pulmonary artery pressure is increased which opens up portions of the lung which are normally not used, thus increasing the capacity of the lungs to absorb oxygen. For the first week or so, the cardiac output increases to increase the level of oxygen delivered to the tissues. The body also begins to increase the production of red blood cells. Other changes include the increase of an enzyme (DPG) which, in-turn, facilitates the release of oxygen from the blood and increase the numbers of capillaries within the muscle to better facilitate the exchange of blood with the muscle.
Tissue hypoxia is caused by the body's inability to obtain or utilize an adequate supply of oxygen. Under normal circumstances, there are three main ways by which this can occur. An individual can breathe a gas mixture in which the percentage of oxygen in the inspired air is insufficient to support adequate cellular respiration. This type of hypoxia (hypoxic hypoxia) can be found in situations where gases such as nitrogen or carbon dioxide are present in higher than normal concentrations relative to air at sea level, thereby displacing oxygen in the gas mixture. Breathing a gas mixture that contains approximately the same percentages of gases as found at sea level, but where the total pressure of the gas mixture is reduced causes a second form of hypoxia (hypobaric hypoxia). This is the situation encountered in altitude exposures. Finally, a third form of hypoxia (histiotoxic hypoxia) is caused by certain toxins (e.g. carbon monoxide, cyanide) that interfere with the body's utilization of oxygen at the cellular level.
Physiologically, the response to each of these types of hypoxia is similar as the organism attempts to compensate for the reduced amount of oxygen available for cellular metabolism. The rate and depth of respiration increases and the heart rate also increases. Subjectively, the individual experiences the sensations of shortness of breath and anxiety. If the hypoxia is severe enough, or if compensatory mechanisms cannot be sustained for any reason, other symptoms become apparent. Organs that have a high oxygen demand are affected first. Cognitive processes are impaired, and the subject may experience marked confusion or ataxia. If the hypoxia persists, coma and death result.
Investigators have utilized different mechanisms to study the effects of hypoxia on human physiology. Exposure to hypobaric environments has been the technique most frequently utilized in aviation settings. The military and commercial aviation industry both spend large sums of money annually training aviators to recognize and experience the signs and symptoms of hypoxia. This type of training is accomplished through the use of hypobaric chambers at fixed sites. These chambers have several drawbacks. Because they are expensive to construct and operate, only a limited number of these chambers can be fielded. Despite their relatively large size, however, they are generally too small to incorporate mission simulators into the hypoxic environment. Additionally, any equipment that is placed into the chamber must be extensively tested to ensure that it is compatible with the reduced barometric pressures within the chamber. Some investigators believe that if hypoxia training and flight could be combined, the face validity of the training scenario would be improved, and the overall training benefit would be significantly increased.
Other investigators have utilized mixed-gas hypoxia (i.e., hypoxic hypoxia) for a variety of reasons, most typically to investigate the physiologic effects of breathing gas mixtures containing a reduced percentage of oxygen, and/or an elevated concentration of carbon dioxide. This technique has several drawbacks. Gas mixtures require the ability to accurately blend and compress gases. Premixed gases also require some storage capacity. Typically, several cylinders of gas mixtures are connected in parallel to a manifold, which is in turn connected to the experimental subject. By changing valve settings on the manifold, differing gas mixtures can be administered. Concentrations are, therefore, limited to only those mixtures created before the experiment. Since the gas mixtures are discrete, no intermediate concentrations can be achieved. The gas mixtures can be administered through a conventional breathing apparatus, but the dependence on cylinders of premixed gases outweighs this convenience. However, because these devices also provoke the symptoms of hypoxia, one potentially useful avenue for these devices could be in the simulation of altitude exposure. Experiments have shown that the physical symptoms and performance deficits induced by hypobaric and mixed-gas hypoxia are qualitatively similar.
Certain devices like the present invention have been presented in the literature as being of two fundamental types. The simplest type exhibits a relatively large volume, closed breathing circuit. An experimental subject is connected to the circuit, and breathes off the reservoir, gradually exchanging the gas mixture present in the reservoir with his or her own exhaled gas (re-breathing). Carbon monoxide and water vapor from the subject may or may not be removed from the reservoir, depending on the experimental design. This type of device is limited in several important respects. The rate at which the oxygen in the reservoir is depleted is dependent on the ratio of the subject's minute ventilation volume and the volume of the reservoir. Since this device has no means to replace oxygen in the reservoir, this device cannot maintain a gas mixture at a particular ratio or concentration. The duration of the experiment is therefore limited to the time it takes for oxygen levels in the reservoir to fall to critical levels. Additionally, the concentration of oxygen in the system is constantly changing making interpretation of the results much more challenging.
A more advanced type of re-breathing circuit has been developed that addresses some of the shortcomings of the simple re-breathing loop. In this device, the subject exhales into a mixing loop, and an oxygen sensor monitors the concentration of oxygen in the loop. Computer software compares the actual concentration of oxygen to the expected concentration of oxygen, and oxygen is added to the mixing loop to hold the concentration of oxygen at a preset level. A shortcoming of this system is that carbon dioxide and water vapor must be continuously removed. Volume loss through the absorption of water vapor and carbon dioxide forces the addition of a replacement volume of gas (typically nitrogen) into the circuit. Because this is a re-breathing apparatus, special masks are required for the subject. Masks are connected to the re-breathing loop by two flexible hoses. Because of the weight of the one-way valve system required, and the weight of the hoses, this apparatus is cumbersome to the subject, and is not well suited for operation in small or confined spaces.
Examples of some of these and similar devices are as follows: Gamow (U.S. Pat. No. 5,398,678) discloses a portable chamber to simulate higher altitude conditions by increasing the pressure within the chamber above that of the ambient pressure, whereas the present invention is practiced in isobaric conditions; Lane (U.S. Pat. No. 5,101,819) teaches a method of introducing nitrogen into a flight training hypobaric chamber (not as in the isobaric conditions of the present invention) to simulate the lower oxygen concentrations at higher altitudes for fighter pilots; Kroll (U.S. Pat. No. 5,988,161) teaches a portable re-breathing device using increasing levels of carbon dioxide to displace oxygen and used to acclimate individuals to higher altitudes, whereas the present invention does not employ this use of exhaled gases (re-breathing) to displace the oxygen; Koni, et al. (U.S. Pat. No. 4,345,612) discloses an apparatus for delivery of a regulated flow of anesthetic gases but uses flow rate input data (not direct measurement of the mixed gases as in the present invention) to control release of gases and is not designed to allow for dynamic conditions; Lampotang, et al. (U.S. Pat. No. 6,131,571) also teaches a device for delivery of anesthetic gases but is more concerned with improved mixing of the gases and maintenance of proper pressure (operating as a ventilator) and is fundamentally different from the present invention, again, in both application and operation (pressure differentials, not direct measurement of mixed gases, is the means for computer control and is utilized to maintain proper system volume, not gas concentrations as in the present invention); and, finally, Marshall, et al. (U.S. Pat. No. 6,196,051) teaches an apparatus for determining odor levels in gas streams but utilizes a mass flow sensor at the inlet valve to regulate the flow of gases into the mixing chamber (not by direct measurement of chamber gases as in the present invention).